Tephra fountaining, rheomorphism, and spatter flow during emplacement of the Pitigliano Tuffs, Latera caldera, Italy

Abstract

Abstract Toward the end of the main ash-flow phase (ca. 156 ka) of the Latera caldera, central Italy, explosive eruptions near the NW caldera rim produced coarse-grained tephra exposed in > 35 m high faces of the Toscapomice (Casa Collina) quarry. The pyroclastic sequence fills a narrow SW-NE paleovalley and records the near-continuous eruption of a compositionally zoned magma batch. White phonolitic pumice clasts abruptly grade upward to, and partly intrude, black spatter-like pumice fragments, also of phonolite composition. This sequence, in turn, is overlain by densely welded tuff that grades upward to tephriphonolitic scoriae and ash. The upper welded part of this sequence—referred to as the “Vulcanite Complessa di Pitigliano”—has long been among the more enigmatic deposits in the Latera area. This sequence displays evidence for both fall and flow emplacement, and is here interpreted as an ash fountain-fed deposit that locally welded as spatter, yet became sufficiently fluidized in distal parts to behave as a spatter-rich pyroclastic flow. This mechanism requires that the eruption underwent abrupt transition from a vigorously convecting column to a weakly convecting ash fountain, and that this period of explosive activity culminated with unusually high accumulation rates. The rapidly emplaced tephra retained sufficient heat to weld by compaction and undergo rheomorphism close to the vent, yet contained high enough proportions of fluidized dust to have flowed outward along the northern caldera rim and, eventually, to have welded by vapor-phase sintering.